Two-junction photovoltaic devices

ABSTRACT

The present disclosure relates to a photovoltaic (PV) device that includes a first junction constructed with a first alloy and having a bandgap between about 1.0 eV and about 1.5 eV, and a second junction constructed with a second alloy and having a bandgap between about 0.9 eV and about 1.3 eV, where the first alloy includes III-V elements, the second alloy includes III-V elements, and the PV device is configured to operate in a thermophotovoltaic system having an operating temperature between about 1500° C. and about 3000° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/627,837 filed Feb. 8, 2018, the contents of which areincorporated herein by reference in their entirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this disclosure underContract No. DE-AC36-08GO28308 between the United States Department ofEnergy and Alliance for Sustainable Energy, LLC, the Manager andOperator of the National Renewable Energy Laboratory.

SUMMARY

An aspect of the present disclosure is a photovoltaic (PV) device thatincludes a first junction constructed with a first alloy and having abandgap between about 1.0 eV and about 1.5 eV, and a second junctionconstructed with a second alloy and having a bandgap between about 0.9eV and about 1.3 eV, where the first alloy includes III-V elements, thesecond alloy includes III-V elements, and the PV device is configured tooperate in a thermophotovoltaic system having an operating temperaturebetween about 1500° C. and about 3000° C. In some embodiments of thepresent disclosure, the first alloy includes at least two of aluminum,gallium, indium, arsenic, and/or phosphorus. In some embodiments of thepresent disclosure, the second alloy includes at least two of aluminum,gallium, indium, arsenic, and/or phosphorus.

In some embodiments of the present disclosure, the first alloy includesAl_(x)Ga_(y)In_(1-x-y)As_(z)P_(1-z), where 0≤x≤1, 0≤y≤1, and 0≤z≤1 andhas a bandgap of between about 1.1 eV and about 1.5 eV. In someembodiments of the present disclosure, the first alloy includes at leastone of about Al_(0.15)Ga_(0.56)In_(0.29)As and/or aboutGa_(0.5)In_(0.5)As_(0.57)P_(0.43). In some embodiments of the presentdisclosure, the second alloy includes Ga_(w)In_(1-w)As, where 0≤w≤1 andhas a bandgap between about 0.9 eV and about 1.2 eV. In some embodimentsof the present disclosure, the second alloy includes aboutGa_(0.7)In_(0.3)As. In some embodiments of the present disclosure, thefirst alloy includes about Al_(0.15)Ga_(0.56)In_(0.29)As and has abandgap of about 1.2 eV, the second alloy includes aboutGa_(0.7)In_(0.3)As and has a bandgap of about 1.0 eV, and the PV deviceis configured for the operating temperature between about 1900° C. andabout 2100° C.

In some embodiments of the present disclosure, the first alloy includesGaAs having a bandgap of about 1.4 eV. In some embodiments of thepresent disclosure, the second alloy includes Ga_(0.85)In_(0.15)As andhas a bandgap of about 1.2 eV. In some embodiments of the presentdisclosure, the first alloy includes GaAs and has a bandgap of about 1.4eV, the second alloy includes about Ga_(0.85)In_(0.15)As and has abandgap of about 1.2 eV, and the PV device is configured for theoperating temperature between about 2400° C.

In some embodiments of the present disclosure, the PV device may furtherinclude a tunnel junction positioned between the first junction and thesecond junction. In some embodiments of the present disclosure, thetunnel junction may include at least two of gallium, indium, arsenide,and/or antimony. In some embodiments of the present disclosure, the PVdevice may further include a buffer layer positioned between the firstjunction and the second junction. In some embodiments of the presentdisclosure, the buffer layer may be constructed of an alloy thatincludes at least two of gallium, indium, arsenide, and/or antimony. Insome embodiments of the present disclosure, the buffer layer may be acompositionally graded buffer layer comprising between two and 15layers. In some embodiments of the present disclosure, the alloy of thebuffer layer may include Ga_(x)In_(1-x)P and 0.20≤x≤0.51. In someembodiments of the present disclosure, the PV device may further includeat least one of a substrate and/or a buffer layer, where the at leastone of the substrate and/or the buffer layer is configured to be removedfrom the PV device.

An aspect of the present disclosure is a thermophotovoltaic (TPV) systemthat includes an emitter operating at a temperature between about 1900°C. and about 2400° C., and a photovoltaic (PV) device, where the emitteris configured to transfer radiant energy to the PV device, and the PVdevice includes a first junction constructed with a first III-V alloyand having a bandgap between about 1.0 eV and about 1.5 eV, and a secondjunction constructed with a second III-V alloy and having a bandgapbetween about 0.9 eV and about 1.3 eV.

An aspect of the present disclosure is a method that includes operatingan emitter at a temperature between about 1900° C. and about 2400° C. tocreate radiant energy, transferring at least a portion of the radiantenergy to a photovoltaic (PV) device, and converting at least a portionof the transferred radiant energy to electricity, where the PV deviceincludes a first junction constructed with a first III-V alloy andhaving a bandgap between about 1.0 eV and about 1.5 eV, and a secondjunction constructed with a second III-V alloy and having a bandgapbetween about 0.9 eV and about 1.3 eV.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments are illustrated in referenced figures of the drawings.It is intended that the embodiments and figures disclosed herein are tobe considered illustrative rather than limiting.

FIG. 1 illustrates a TPV energy system, according to some embodiments ofthe present disclosure.

FIG. 2A illustrates a thermophotovoltaic (TPV) unit that includes amultijunction photovoltaic device, according to some embodiments of thepresent disclosure.

FIG. 2B illustrates a photovoltaic (PV) device suitable forthermophotovoltaic applications, according to some embodiments of thepresent disclosure.

FIGS. 3A-3D illustrate schematic cross-sections of multijunction PVstacks for use in a PV device suitable for use in thermophotovoltaicsystems, according to some embodiments of the present disclosure.

FIG. 4 illustrates the net power transfer between a 2000° C. tungstenemitter and a 1.2 eV bandgap first junction. 100% absorption above thebandgap is assumed, and 2% parasitic absorption below the bandgap. Thelong wavelength tail extends out to 10 μm (not shown).

FIG. 5 illustrates two-junction PV stack efficiencies under modifiedblackbody emission spectra at temperatures between about 1700° C. andabout 2400° C., according to some embodiments of present disclosure. Thedashed curve highlights the 2000° C. result. The bottom axis indicatesthe second (bottom) junction bandgap, and the dashed lines and rightaxis indicates the first (top) junction bandgap.

FIGS. 6A and 6B illustrate alloy compositions for two junctions TPVdevices, according to some embodiments of the present disclosure.

FIG. 7A illustrates external quantum efficiencies collected for a PVdevice designed for TPV energy systems, according to some embodiments ofthe present disclosure.

FIG. 7B illustrates the current-voltage curves collected for a PV devicedesigned for TPV energy systems, according to some embodiments of thepresent disclosure.

FIG. 8A illustrates external quantum efficiencies collected for a PVdevice designed for TPV energy systems, according to some embodiments ofthe present disclosure.

FIG. 8B illustrates the current-voltage curves collected for a PV devicedesigned for TPV energy systems, according to some embodiments of thepresent disclosure.

REFERENCE NUMBERS 100 thermophotovoltaic (TPV) energy system 110 firsttank 115 cold heat transfer (HT) fluid 120 heat source 125 hot HT fluid130 second tank 140 TPV unit 150 pump 200 photovoltaic (PV) device 210heat transfer fluid channel 220 space 225 emitter 230 outer layer 240multijunction PV stack 250 reflective layer 300 substrate 310 bufferlayer 320 first junction 330 tunnel junction 340 second junction 350handle

DETAILED DESCRIPTION

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, “some embodiments”, etc., indicate that theembodiment described may include a particular feature, structure, orcharacteristic, but every embodiment may not necessarily include theparticular feature, structure, or characteristic. Moreover, such phrasesare not necessarily referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with an embodiment, it is submitted that it is within theknowledge of one skilled in the art to affect such feature, structure,or characteristic in connection with other embodiments whether or notexplicitly described.

As used herein the term “substantially” is used to indicate that exactvalues are not necessarily attainable. By way of example, one ofordinary skill in the art will understand that in some chemicalreactions 100% conversion of a reactant is possible, yet unlikely. Mostof a reactant may be converted to a product and conversion of thereactant may asymptotically approach 100% conversion. So, although froma practical perspective 100% of the reactant is converted, from atechnical perspective, a small and sometimes difficult to define amountremains. For this example of a chemical reactant, that amount may berelatively easily defined by the detection limits of the instrument usedto test for it. However, in many cases, this amount may not be easilydefined, hence the use of the term “substantially”. In some embodimentsof the present disclosure, the term “substantially” is defined asapproaching a specific numeric value or target to within 20%, 15%, 10%,5%, or within 1% of the value or target. In further embodiments of thepresent disclosure, the term “substantially” is defined as approaching aspecific numeric value or target to within 1%, 0.9%, 0.8%, 0.7%, 0.6%,0.5%, 0.4%, 0.3%, 0.2%, or 0.1% of the value or target.

As used herein, the term “about” is used to indicate that exact valuesare not necessarily attainable. Therefore, the term “about” is used toindicate this uncertainty limit. In some embodiments of the presentdisclosure, the term “about” is used to indicate an uncertainty limit ofless than or equal to ±20%, ±15%, ±10%, ±5%, or ±1% of a specificnumeric value or target. In some embodiments of the present disclosure,the term “about” is used to indicate an uncertainty limit of less thanor equal to ±1%, ±0.9%, ±0.8%, ±0.7%, ±0.6%, ±0.5%, ±0.4%, ±0.3%, ±0.2%,or ±0.1% of a specific numeric value or target.

The present disclosure relates to new designs for multijunction (e.g.two junction) photovoltaic devices, which in some embodiments, may beused in thermophotovoltaic (TPV) storage applications. In TPV energysystems, the energy is provided by an emitter (for example, a materialsuch as a metal or semiconductor), which is heated to a workingtemperature T_(w) by a heat source. The TPV emitter can be described asa blackbody having an emission spectrum characteristic of T_(w) and ofthe emissivity of the emitter material, and which transfers energy byradiant heat transfer to a photovoltaic device, which converts theradiant energy to electricity. In some embodiments of the presentdisclosure, a TPV energy system and its associated photovoltaic devicedesigns and emitter may operate at a T_(w) that is between about 1500°C. and about 3000° C., or between about 1900° C. and about 2400° C.,much higher than typical TPV applications which are often less than1200° C. As a result of the higher T_(w), the target bandgaps of thephotovoltaic device are also higher than they would be for a lowerT_(w). In some embodiments of the present disclosure, the targetbandgaps may be between about 1.1 eV and about 1.4 eV for the topjunction (the first junction to receive light) and between about 0.9 eVand about 1.2 eV for the bottom junction (the second junction to receivelight; e.g. the remaining light that has passed through the topjunction).

FIG. 1 illustrates a TPV energy system 100, according to someembodiments of the present disclosure. In this example, the TPV energysystem 100 has a thermophotovoltaic unit 140 in which radiant heattransfer occurs from an emitter (not shown) to at least one photovoltaicdevice (not shown), resulting in the conversion of thermal energy toelectricity. (The emitter and photovoltaic device are explained in moredetail in FIG. 2A.) As shown in FIG. 1, the TPV energy system 100 may beconfigured as a circuit through which a heat transfer (HT) fluid iscirculated using a pump 150. Although FIG. 1 only illustrates one pump150, in some embodiments of the present disclosure, one or more pumpsmay be utilized to move the HT fluid between the various unit operationsof the TPV energy system 100. This may, for example, depend on thedesign of one or more of the unit operations. Examples of HT fluidssuitable for the present application include liquid silicon or liquidiron, among other possibilities. The HT fluid may be stored in a firsttank 110, where the HT fluid is at some temperature that is less thanthe targeted T_(w) by at least about 500° C. When needed, thisrelatively “cold” HT fluid 115A may be circulated to a heat source 120by the pump 150 where the cold HT fluid 115A is heated to a temperaturethat is greater than or equal to the targeted T_(w), resulting in arelatively “hot” HT fluid 125A. As described herein, the hot HT fluidtemperature is targeted for a T_(w) (or range of temperatures) thatresults in the emitter radiating wavelengths of light that can beabsorbed with high efficiencies by the photovoltaic device. The hot HTfluid 125A may be directed to a second tank 130 for storage until neededand/or the hot HT fluid 125A may be fed directly to the downstreamthermophotovoltaic (TPV) unit 140. After heat is transferred from thehot HT fluid 125B to the photovoltaic device (not shown) positionedwithin the thermophotovoltaic unit 140, the resultant cold HT fluid(115B and/or 115C) may then be transferred back to the first tank 110 bythe pump 150.

FIG. 2A illustrates a TPV unit 140, according to some embodiments of thepresent disclosure. In this example, the TPV unit 140 includes a stackof multiple PV devices (200A and 200B) positioned centrally within aspace 220 formed by the walls, in this case four walls, of an emitter225. The walls of the emitter 225 may include a plurality of HT fluidchannels (e.g. 210A and 210B), through which the hot HT fluid 125 may bedirected (e.g. by the pump 150). In some embodiments of the presentdisclosure, the HT fluid channels 210 may form the walls of the emitter.In some embodiments of the present disclosure, the HT fluid channels 210may be positioned adjacent to at least one external solid surface and/oran internal solid surface. Although this example

In some embodiments of the present disclosure, any of the individualunit operations (e.g. pump, tanks, piping, and/or TPV unit, etc.) of aTPV energy system 100, may utilize 553 metallurgical grade (98.5% pure)silicon as the HT fluid (115 and 125). The liquid silicon HT fluid maybe stored in the first tank 110 at about 1900° C. Transfer of thesilicon HT fluid (115 and 125) through the TPV energy system 100 may beachieved using an all graphite seal-less sump pump 150. In someembodiments, such a pump 150 may transfer the cold HT fluid 115A to aheat source 120 constructed of a series of pipes that are externallyirradiated by electrically powered graphite heaters, resulting in hot HTfluid 125A of liquid silicon at a temperature of up to about 2400° C.This hot HT fluid 125A may then be stored in the second tank 130. Insome embodiments of the present disclosure, the tanks (110 and 130) maybe relatively large, with diameters on the order of about 10 meters,which allows the surface area to volume ratio to be small enough thatenergy losses to the environment may be limited to less than 1% of theenergy stored per day.

When electricity production is desired, the hot HT fluid 125B, e.g.liquid silicon at about 2400° C., may be pumped out of the second tank130 and through the TPV unit 140. Referring again to FIG. 2A, a TPV unit140 may include an array of graphite HT fluid channels 210 that may becovered in tungsten foil (not shown). The tungsten foil may provide alower vapor pressure barrier between the graphite HT fluid channels 210and the PV device(s) 200. In some embodiments of the present disclosure,the PV device(s) 200 may be mounted to an actively cooled (e.g. usingcirculated cooling water) block that maintains the PV device(s) 200 at atemperature between about 20° C. and about 50° C. Lower PV operatingtemperatures may increase the power conversion efficiency of the PVdevice(s) 200. Thus, at least one of the graphite HT fluid channels 210and/or the tungsten foil may act as a photon emitter, emitting light tothe PV device(s) 200, which converts a fraction of this radiant energyto electricity. As the hot liquid silicon HT fluid 125 passes throughthe HT fluid channels 210 of the TPV unit 140, at least a portion ofthermal energy contained in the hot HT fluid is transferred by radiantheat transfer to the PV device(s) 200 and converted to electricity. Theresultant cold HT fluid 115C may then be returned to the first tank 110to be stored until the cycle needs to be repeated (e.g. depending onelectricity demand).

Although FIGS. 1 and 2A illustrate a TPV energy system 100 and TPV unit140 that utilize a heat transfer fluid to heat an emitter 225, it isnoted that other TPV energy systems, TPV units, and emitters thatutilize other heating systems are possible and are considered within thescope of the present disclosure. For example, a heat source may beintegrated directly into an emitter, thus completely eliminating theneed for a HT fluid and the associated unit operations, e.g. storagetanks, piping, pump, etc.

FIG. 2B illustrates a PV device 200 suitable for a TPV unit 140,according to some embodiments of the present disclosure. In thisexample, the PV device 200 may include a multijunction PV stack 240positioned between an outer layer 230 (e.g. a protective layer and/oranti-reflective layer) and a reflective layer 250, where the outer layer230 is positioned to first receive the light from the emitter 225 (e.g.the outer layer 230 is positioned closest to emitter 225). As usedherein, the term “stack” refers to a device that includes two or morelayers of differing composition, thicknesses, etc., that are positionedone on top of the other relative to a reference axis that is essentiallyperpendicular to the planes of each of the layers. The PV device 200which converts photons radiated from the emitter 225 may have asignificant role in the TPV energy system's thermal energy toelectricity conversion efficiency. The energy in a photon (eV_(photon))incident on a PV device 200 can suffer several types of losses, whichare very strongly dependent on the spectrum of light, and on the designof the PV device itself. The most significant is the voltage loss, wherean incident photon is absorbed by the PV device 200 and converted intoelectrical current, at a cell open-circuit voltage V_(OC)<V_(photon).This energy loss E_(loss)=eV_(photon)−eV_(OC) can be partitioned intotwo individual losses related to the junction bandgap (E_(g)):E_(loss)=(eV_(photon)−E_(g))+(E_(g)−eV_(OC)).

The first loss, referred to as thermalization loss, arises because thethermally-radiated spectrum contains a wide range of photon energies,and the energy above the bandgap is lost. To mitigate this loss, in someembodiments of the present disclosure, a PV device 200 that includes amultijunction PV stack 240 may be used, for example a two junction PVstack 240, which has been designed to provide two bandgaps chosen tooptimally convert a specific portion of the light spectrum thatcorresponds to a specific T_(w) and/or T_(w) range. The E_(g)−eV_(OC)penalty is proportionally smaller for higher bandgap materials than forlower bandgap materials. Thus, for some of the embodiments describedherein, the relatively high emitter temperatures (T_(w)) used, e.g.between about 1500° C. and about 3000° C., may generate correspondinglyhigh-energy photons for which relatively high-bandgap multijunction PVstacks 240 are suitable, thus minimizing and/or eliminating theE_(g)−eV_(OC) penalty. Similarly, the high T_(w) temperatures usedherein may also result in a very high flux of photons, which increaseseV_(OC), further suppressing this penalty. A type of absorption lossalso occurs at the system level and results from the free carrierabsorption of sub-bandgap photons, either at the reflective layer 250 orin the semiconductor material itself used to construct the multijunctionPV stack 240. A reflective layer 250 should be as reflective aspossible, so that unusable sub-bandgap photons can be reflected backthrough the PV device 200 for another chance at being absorbed and/orreflected back to the emitter 225.

FIG. 3A illustrates a multijunction PV stack 240, in this case a twojunction PV stack, according to some embodiments of the presentdisclosure. As described herein, some embodiments of multijunction PVstacks 240 may include some or all of the individual elements shown inFIG. 3A. Thus, in some embodiments of the present disclosure, amultijunction PV stack 240 may include a first junction 320 and secondjunction 340, where the first junction 320 is positioned to receivelight first from the emitter 225 and the second junction 340 ispositioned to receive light second (e.g. the light that was transmittedthrough the first junction 320). Thus, the first junction 320 may bedesigned to have a first bandgap, and the second junction 340 may bedesigned to have a second bandgap, where the first bandgap is largerthan the second bandgap. In some embodiments of the present disclosure,a multijunction PV stack 240 may include a tunnel junction 330 that ispositioned between the first junction 320 and the second junction 340. Atunnel junction is a type of diode with degenerate doping on both sides.Carriers can cross the junction by means of quantum mechanicaltunneling. The tunnel junction provides electrical conduction between ap-type layer on one side and an n-type layer on the other side.

In addition, a multijunction PV device 240 may include a first bufferlayer 310A positioned between the first junction 320 and the secondjunction 340. In some embodiments of the present disclosure, a firstbuffer layer 310A may be positioned between the tunnel junction 330 andthe second junction 340. A first buffer layer 310A, and a buffer layerin general, may be a series of thin layers with progressively larger (orsmaller) lattice constants. Changing the lattice constant induces strainin the semiconductor, which can lead to threading dislocations thatdegrade the material. The first buffer layer 310A helps to relieve thestrain in a controlled manner, to enable growth of the second junction340 with high quality. In some embodiments of the present disclosure, amultijunction PV stack 240 may include a substrate 300 and a secondbuffer layer 310B, where the second buffer layer 310B is positionedbetween the substrate 300 and the first junction 320. A substrate 300may be a thick semiconductor on which the epitaxial layers aredeposited. Substrates are typically polished, are purchased from amanufacturer, and are typically 300-650 μm thick. Examples of commonsubstrates include GaAs, InP, InAs, GaSb, Ge and Si. Referring again toFIG. 3A, a multijunction PV stack 240 may include a handle 350, wherethe second junction 340 is positioned adjacent to the handle. The handleis a material to which the epitaxial layers are secured, to providemechanical stability after the substrate is etched away. Examplesinclude silicon, glass and flexible foil, although other handles arepossible. The handle can be electrically insulating or conductive.

Referring again to FIG. 3A, the multijunction stack 240 may be grown bymetalorganic vapor phase epitaxy (MOVPE), although other techniques suchas molecular beam epitaxy (MBE) or hydride vapor phase epitaxy (HVPE)are also possible. The stack may be grown inverted on the substrate 300,starting with the first buffer layer 310B and finishing with the secondjunction 340. MOVPE growth typically occurs at temperatures betweenabout 550° C. and about 800° C. and at pressures between about 0.01 Torrand 760 Torr. After growth, a back contact (not shown) may be depositedon the second junction 340, the multijunction PV stack 240 secured to ahandle 350, and the substrate 300 may be removed. In some embodiments ofthe present disclosure, the first buffer layer 310B may also be removed.Front metal contacts (not shown) may then be deposited, the individualdevices may be isolated by either wet-chemical etching or reactive ionetching, and finally an anti-reflection coating (not shown) may bedeposited on the front surface; e.g. on the outer surface of the firstjunction 320.

FIG. 3B illustrates an example of a multijunction PV stack 240,according to some embodiments of the present disclosure. In thisexample, the multijunction PV stack 240 has two junctions, a firstjunction 320 and a second junction 340 with a tunnel junction 330positioned between the first junction 320 and the second junction 340.FIG. 3B also shows a multijunction PV stack 240 having a handle 350 toprovide mechanical support for handling after the deposition processesto make the multijunction PV stack 240 are completed. In this example,the substrate 300 is used as a starting surface for depositing at leastone buffer layer 310, whose final layer is lattice-matched to the firstjunction 320. This substrate 300, as well as the buffer layer 310itself, may be removed (as indicated by the dotted lines) from themultijunction PV stack 240, resulting in the final device, a monolithicmultijunction PV stack 240 having a first junction 320, a tunneljunction 330, a second junction 340, and a handle 350. Once removed, thesubstrate 300 and/or the buffer layer 310 may be recycled and/or reusedto make additional multijunction PV stacks 240.

In some embodiments of the present disclosure, a multijunction PV stack240 similar to that shown in FIG. 3B may utilize a substrate 300constructed of at least one of GaAs and/or germanium. A first junction320 may be constructed of an alloy that includes at least one ofAl_(x)Ga_(y)In_(1-x-y)As, where 0≤x≤1 and 0≤y≤1, and/or an alloy thatincludes Ga_(y)In_(1-y)As_(z)P_(1-z), where 0≤y≤1 and 0≤z≤1. In general,the first junction 320 may include any III-V alloy composition thatprovides a bandgap between 1.1 eV and 1.5 eV optimized for absorbinglight radiated by an emitter having an operating temperature, T_(w),between about 1700° C. and about 2400° C. For example, a multijunctionPV stack 240 similar to that shown in FIG. 3B may have a first junction320 that includes at least one of Al_(0.15)Ga_(0.55)In_(0.3)As and/orGa_(0.5)In_(0.5)As_(0.57)P_(0.43). The buffer layer 310 may include acompositionally graded buffer layer, having between 2 and 15 layers andcomposition of Ga_(x)In_(1-x)P where x is varied between 0.51 and 0.20,or similarly Al_(x)Ga_(y)In_(1-x-y)As, where the In-content is varied,to provide a final layer of the buffer that is lattice-matched to thefirst junction 320. In general, the graded buffer layer may include anyIII-V alloy composition that accesses the lattice-constant of the firstjunction.

Referring again to FIG. 3B, the second junction 340 may be constructedof an alloy that includes Ga_(y)In_(1-y)As where 0≤y≤1. In general, thesecond junction 340 may include any III-V alloy composition thatprovides a bandgap between 0.9 eV and 1.3 eV optimized for absorbinglight radiated by an emitter having an operating temperature, T_(w),between about 1700° C. and about 2400° C. For example, a multijunctionPV stack 240 similar to that shown in FIG. 3B may have a second junction340 that includes Ga_(0.7)In_(0.3)As. Further, as shown in FIG. 3B, thisexemplary multijunction PV stack 240 may include a tunnel junction 330positioned between the first junction 320 and the second junction 330,where the tunnel junction 330 is constructed of an alloy that includes,for example, GaInAs and/or GaAsSb. This exemplary multijunction PV stack240 also includes a front contact layer of the same lattice constant asthe first junction 320, for example an alloy of GaInAs or GaInAsN.

FIG. 3C illustrates another example of a multijunction PV device 240suitable for TPV energy systems, according to some embodiments of thepresent disclosure. In this example, the multijunction PV stack 240 hastwo junctions, a first junction 320 and a second junction 340 with atunnel junction 330 positioned between the first junction 320 and thesecond junction 340. In addition, this example includes a first bufferlayer 310A positioned between the tunnel junction 320 and the secondjunction 340. This first buffer layer 310A is provided to overcome thelattice-mismatch between the first junction 320 and the second junction340. FIG. 3C also shows a multijunction PV stack 240 having a handle 350to provide mechanical support for handling after the depositionprocesses to make the multijunction PV stack 240 are completed. In thisexample, a substrate 300 is used as a starting surface for depositing asecond buffer layer 310B whose final layer is lattice-matched to thefirst junction 320. This substrate 300, as well as the second bufferlayer 310B itself, may be removed (as indicated by the dotted lines)from the multijunction PV stack 240, resulting in the final device, amonolithic multijunction PV stack 240 having a first junction 320, atunnel junction 330, a first buffer layer 310A, a second junction 340,and a handle 350. Once removed, the substrate 300 and/or the secondbuffer layer 310B may be recycled and/or reused to make additionalmultijunction PV stacks 240.

In some embodiments of the present disclosure, a multijunction PV stack240 similar to that shown in FIG. 3C may utilize a substrate 300constructed of at least one of GaAs and/or germanium. A first junction320 may be constructed of an alloy that includesAl_(x)Ga_(y)In_(1-x-y)As, where 0≤y≤1. In general, the first junction320 may include any III-V alloy composition that provides a bandgapbetween 1.1 eV and 1.3 eV optimized for absorbing light radiated by anemitter having an operating temperature, T_(w), between about 1700° C.and about 2200° C. For example, a multijunction PV stack 240 similar tothat shown in FIG. 3C may have a first junction 320 that includes atleast one of Ga_(0.85)In_(0.15)As. The first buffer layer 310A mayinclude a compositionally graded buffer layer, having between 2 and 15layers and a composition of Ga_(x)In_(1-x)P where x is varied between0.51 and 0.20 to provide a final layer that is lattice-matched to thesecond junction 340. Similarly, the second buffer layer 310B may includea compositionally graded buffer layer, having between 2 and 15 layersand composition of Ga_(x)In_(1-x)P where x is varied between 0.51 and0.20 to provide a final layer of the buffer that is lattice-matched tothe first junction 320.

Referring again to FIG. 3C, the second junction 340 may be constructedof an alloy that includes Ga_(y)In_(1-y)As where 0≤y≤1. In general, thesecond junction 340 may include any III-V alloy composition thatprovides a bandgap between 0.9 eV and 1.1 eV optimized for absorbinglight radiated by an emitter having an operating temperature, T_(w),between about 1700° C. and about 2200° C. For example, a multijunctionPV stack 240 similar to that shown in FIG. 3C may have a second junction340 that includes Ga_(0.7)In_(0.3)As. Further, as shown in FIG. 3C, thisexemplary multijunction PV stack 240 may include a tunnel junction 330positioned between the first junction 320 and the second junction 330,where the tunnel junction 330 is constructed of an alloy that includesat least one of GaInAs and/or GaAsSb. This exemplary multijunction PVstack 240 also includes a front contact layer of the same latticeconstant as the first junction 320, for example an alloy of GaInAs orGaInAsN.

FIG. 3D illustrates another example of a multijunction PV device 240suitable for TPV energy systems, according to some embodiments of thepresent disclosure. In this example, the multijunction PV stack 240 hastwo junctions, a first junction 320 and a second junction 340 with atunnel junction 330 positioned between the first junction 320 and thesecond junction 340. In addition, this example includes a buffer layer310 positioned between the tunnel junction 320 and the second junction340. This buffer layer 310 is needed to overcome the lattice-mismatchbetween the first junction 320 and the second junction 340. FIG. 3D alsoshows a multijunction PV stack 240 having a handle 350 to providemechanical support for handling the stack after the deposition processesto make the multijunction PV stack 240 are completed. In this example, asubstrate 300 is used as a starting surface for depositing the firstjunction 320. This substrate 300 may be removed (as indicated by thedotted lines) from the multijunction PV stack 240, resulting in thefinal device, a monolithic multijunction PV stack 240 having a firstjunction 320, a tunnel junction 330, a buffer layer 310, a secondjunction 340, and a handle 350. Once removed, the substrate 300 may berecycled and/or reused to make additional multijunction PV stacks 240.

In some embodiments of the present disclosure, a multijunction PV stack240 similar to that shown in FIG. 3D may utilize a substrate 300constructed of at least one of GaAs and germanium. A first junction 320may be constructed of an alloy that includes at least one ofAl_(y)Ga_(1-y)As, where 0≤y≤1; Al_(x)Ga_(y)In_(1-x-y)P, where 0≤x≤1 and0≤y≤1 and the alloy is lattice-matched to the substrate 300; and/orGa_(x)In_(1-x)As_(y)P_(1-y) where 0≤x≤1 and 0≤y≤1 and the alloy islattice-matched to the substrate 300. In general, the first junction 320may include any III-V alloy composition that provides a bandgap between1.3 eV and 1.5 eV optimized for absorbing light radiated by an emitterhaving an operating temperature, T_(w), between about 2000° C. and about2500° C. The buffer layer 310 may include a compositionally gradedbuffer layer, having between 2 and 15 layers and composition ofGa_(x)In_(1-x)P where x is varied between 0.51 and 0.20, or similarlyAl_(x)Ga_(y)In_(1-x-y)As, where the In-content is varied, to provide afinal layer of the buffer that is lattice-matched to the second junction340. In general, the graded buffer layer may include any III-V alloycomposition that accesses the lattice-constant of the second junction.Referring again to FIG. 3D, the second junction 340 may be constructedof an alloy that includes Ga_(y)In_(1-y)As where 0≤y≤1. In general, thesecond junction 340 may include any III-V alloy composition thatprovides a bandgap between 1.1 eV and 1.3 eV optimized for absorbinglight radiated by an emitter having an operating temperature, T_(w),between about 2200° C. and about 2400° C. For example, a multijunctionPV stack 240 similar to that shown in FIG. 3D may have a second junction340 that includes Ga_(0.85)In_(0.15)As. Further, as shown in FIG. 3D,this exemplary multijunction PV stack 240 may include a tunnel junction330 positioned between the first junction 320 and the second junction330, where the tunnel junction 330 is constructed of alloys that includeat least one of GaAs and/or AlGaAs. In this exemplary embodiment, thefront contact layer has the same lattice constant as the substrate 300and could be constructed of alloys including at least one of GaAs and/orGaInAsN.

The graded layers 310 described above typically have between 2 and 15layers and composition of Ga_(x)In_(1-x)P where x is varied between 0.51and 0.20. Other alloys are also possible, for exampleAl_(x)Ga_(y)In_(1-x-y)As, where the In-content varies between 0 and 0.4,or Al_(x)Ga_(1-x)As_(y)Sb_(1-y), where y varies between 0 and 0.4. Ingeneral, the alloy the graded buffer layer may include any III-V alloycomposition that accesses the lattice-constant of the mismatchedjunction.

For all multijunction PV stacks 240 described in the paragraphs above,the back reflector may be deposited between the second junction 340 andthe handle 350. For the multijunction PV cell 240 to operate efficientlyas a TPV converter, the reflector should have a very high averagereflectance (>95%) over all wavelengths from the bandgap (in wavelengthunits) of the second junction 340 to ˜10 μm. The bandgap E_(g) inwavelength units of μm can be calculated as 1.24/E_(g)(eV). For example,GaAs has a bandgap of 1.42 eV, or equivalently 0.87 μm.

In some embodiments of the present disclosure, a multijunction PV stack240 for a TPV energy system 100 may be designed for an emission spectrumproduced by an emitter 225 operating at a T_(w) of about 2000° C. withsome modifications for parasitic absorption effects, as shown in FIG. 4.For this example, the multijunction PV stack 240 was modeled assumingGaAs-like absorption coefficients, a bandgap-voltage offset W_(oc)(=E_(g)/q−V_(oc))=0.4 at 1000 W/m², and with thinning of the first (top)junction as necessary for current matching. FIG. 5 shows theefficiencies for two junction PV stacks 240 under the 2000° C. spectrum(long-dashed line) as well as at other temperatures (solid lines). Thex-axis shows the second (bottom) junction bandgap. The short-dashedlines show the corresponding first (top) junction bandgap on theright-vertical axis, and the model indicates that the optimum bandgapsare about 1.2 eV and about 1.0 eV for the 2000° C. emission spectrum,for the first junction and second junction respectively.

To achieve a high-performance multijunction PV stack 240 with thesebandgaps, some embodiments of the present disclosure include an invertedlattice-mismatched photovoltaic stack 240 on a GaAs substrate 300. Thegrowth may have a single compositionally graded buffer layer (CGB) tothe appropriate lattice constant, and then a contact layer, first (top)junction, tunnel junction, second (bottom) junction, and back contact.During processing, the entire CGB layer may be etched away, so that thetransparency of the CGB is irrelevant. This distinguishes some of theembodiments described herein from other inverted metamorphic (IMM)multijunction PV stacks where the CGB remains part of the finalstructure and therefore should be transparent to long wavelength light.Removing the CGB also greatly reduces the free carrier absorption in thestructure, which improves the efficiency of the TPV energy system 100.

FIG. 6A shows alloy compositions for an exemplary design for amultijunction PV stack 240, according to some embodiments of the presentdisclosure. The second (bottom) junction may be constructed ofGa_(0.7)In_(0.3)As and the first (top) junction may be constructed ofAl_(0.15)Ga_(0.55)In_(0.3)As, both with a nominal relaxed latticeconstant of 5.775 Å. This results in a ˜2.1% mismatch relative to theGaAs substrate. A CGB, grown between the substrate and the firstjunction, to address this lattice mismatch, can be at least one of aGaInP-based alloy and/or an AlGaInAs-based alloy. A window layer (notshown in the figures) may be provided having a composition, e.g.AlGaInP, that provides a suitable lattice constant, e.g. of about 5.775Å. A tunnel junction may be positioned between the first junction andthe second junction, using a suitable alloy, for example, at least oneof GaInAs and/or GaAsSb. A cross-sectional schematic of a design for amultijunction PV stack 240 utilizing these exemplary alloys for thefirst junction and the second junction is shown in FIG. 3B, which isdescribed in detail above. The AlGaInAs alloy in the design of FIG. 6Aand FIG. 3B could be replaced with a quaternary alloy ofGa_(0.5)In_(0.5)As_(0.57)P_(0.43) that has a bandgap of ˜1.2 eV and islattice matched to the 1.0 eV bandgap of a Ga_(0.7)In_(0.3)As secondjunction.

FIG. 6B illustrates another possible alloy composition, according tosome embodiments of the present disclosure. In this example, a first(top) junction may be constructed of an alloy of Ga_(0.85)In_(0.15)Ashaving a bandgap of about 1.2 eV. The second (bottom) junction may beconstructed of an alloy of Ga_(0.7)In_(0.3)As having a bandgap of about1.0-eV. A cross-sectional schematic of a design for a multijunction PVstack 240 utilizing these exemplary alloys for the first junction andthe second junction is shown in FIG. 3C. For this particular design of amultijunction PV stack, two CGB layers and two GaInAs junctions ofdifferent compositions may be utilized. During processing, the firstCGB, positioned between the substrate and the first junction, may beetched away, while the second CGB positioned between the first junctionand the second junction remains part of the multijunction PV stack 240.In this design, the transparency of the first CGB is irrelevant, but thesecond CGB should be transparent to light with energy between thebandgaps of the first and second junctions.

The tandem cell efficiencies summarized in FIG. 5 show the bandgapcombination of 1.4 eV/1.2 eV is very nearly as efficient as 1.2/1.0 eVfor the 2000° C. emitter temperature which is the focus of someembodiments of the present disclosure, at least partly because it iseasier to manufacture in practice because of the smaller latticemismatch. FIG. 3D shows another embodiment of a design to achieve thatbandgap combination. Also, as shown in FIG. 5, other bandgapcombinations may be more appropriate for other emission temperatures.The architecture described here and illustrated in FIGS. 6A and 6B andFIGS. 3A-3D may be modified as appropriate, grading to a differentlattice constant and adjusting the compositions of the (Al)GaInAs,GaInAsP and/or GaInAs alloys. For example, in some embodiments of thepresent disclosure, a PV stack may be constructed with the generalarchitecture of that illustrated in FIG. 3B, however, with the bandgapsof the PV stack illustrated in FIG. 3D. For this example, the PV stackmay have a CGB with a lattice constant of about 5.714 Å, correspondingto the lattice constant of a second junction constructed of GaInAs andhaving a bandgap of about 1.2 eV; then a front contact layer; then a 1.4eV first junction constructed of at least one of an alloy of aboutAl_(0.15)Ga_(0.7)In_(0.15)As and/or aboutGa_(0.63)In_(0.37)As_(0.53)P_(0.47); then a lattice-mismatched tunneljunction; and then a second junction constructed of an alloyapproximating Ga_(0.85)In_(0.15)As, having a bandgap of about 1.2 eV.Similarly, as in FIG. 3B, the CGB would be etched away duringprocessing. This bandgap combination of 1.4/1.2 eV is optimized for anemitter operating at a temperature, Tw, of about 2400°.

FIGS. 7A and 7B illustrate the measured quantum efficiencies and thecurrent-voltage curves respectively for a PV stack constructed for a TPVenergy system, according to some embodiments of the present disclosure.This exemplary PV stack is similar to the stack generally illustrated inFIG. 3B and described above. This PV stack, having the identifying nameof “MR519”, was designed and constructed to have a first junction with afirst bandgap of about 1.2 eV and a second junction with a secondbandgap of about 1.0 eV. MR519 was constructed as an invertedmetamorphic (IMM) two-junction PV device having a compositionally gradedbuffer (CGB) layer and the two junctions at the same lattice-mismatchedlattice constant. The first junction was constructed with an alloy ofabout Al_(0.15)Ga_(0.56)In_(0.29)As and the second junction wasconstructed of an alloy approximating Ga_(0.7)In_(0.29)As. Bothjunctions were a traditional n/p configuration, with a thin emitter anda thick base. Both cells were confined on the front and back sides byGa_(0.23)In_(0.77)P, doped with silicon on the front and zinc on theback. A GaAsSb/GaInAs tunnel junction was positioned between the twojunctions.

A difference between this PV stack and the PV stack illustrated in FIG.3B is that the CGB layer was not removed and remained a part of thefinal device structure of MR519. Thus, in MR519, the front contact layer(not shown) was grown outside the CGB (e.g. it was grown lattice matchedto the substrate, before the CGB). Both lattice-mismatched junctionswere at the same lattice constant, with only a tunnel junctionpositioned between them. FIG. 7A illustrates the measured externalquantum efficiency, which correspond to bandgaps of about 1.23 eV andabout 1.0 eV for the first junction and second junction, respectively.These bandgaps are nearly optimal for a TPV system operating with anemitter temperature, T_(w), between about 1900° C. and about 2100° C.FIG. 7B illustrates the current-voltage curves measured at highconcentration, taken on a flash simulator, for PV stack MR519. Thesecurrent-voltage curves illustrate that there was no evidence of tunneljunction failure up to the highest measured current density, and thatthe voltage increased with current density.

FIGS. 8A and 8B illustrate the quantum efficiencies and thecurrent-voltage curves respectively for a PV stack constructed for a TPVenergy system, according to some embodiments of the present disclosure.This exemplary stack is similar to the stack generally illustrated inFIG. 3D and described above. This PV stack, having the identifying nameof “MR182”, was designed and constructed to have a first junction with afirst bandgap of about 1.4 eV and a second junction with a secondbandgap of about 1.2 eV. MR182 was constructed as an invertedmetamorphic (IMM) two-junction PV device having a first junctionconstructed of GaAs with a bandgap of about 1.4 eV, a compositionallygraded buffer (CGB) layer and then a second junction constructed of analloy approximating Ga_(0.85)In_(0.15)As with a bandgap of about 1.2 eV.Both junctions had a traditional n/p configuration, with a thin emitterand a thick base. Both junctions were confined on the front and backsides by appropriate layers. A GaAs/AlGaAs tunnel junction separated thetwo junctions and was situated between the GaAs cell and the CGB. FIG.8A illustrates the measured external quantum efficiency, whichcorresponds to bandgaps of about 1.41 eV and about 1.18 eV for the firstjunction and second junction, respectively. These bandgaps are nearlyoptimal for a TPV energy system operating with an emitter temperature,T_(w), of about 2400° C. FIG. 8B illustrate the current-voltage curvesmeasured at high concentration, taken on a flash simulator, for PV stackMR182. These current-voltage curves illustrate that there was noevidence of tunnel junction failure up to the highest measured currentdensity, and that the voltage increased with current density.

The foregoing discussion and examples have been presented for purposesof illustration and description. The foregoing is not intended to limitthe aspects, embodiments, or configurations to the form or formsdisclosed herein. In the foregoing Detailed Description for example,various features of the aspects, embodiments, or configurations aregrouped together in one or more embodiments, configurations, or aspectsfor the purpose of streamlining the disclosure. The features of theaspects, embodiments, or configurations, may be combined in alternateaspects, embodiments, or configurations other than those discussedabove. This method of disclosure is not to be interpreted as reflectingan intention that the aspects, embodiments, or configurations requiremore features than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment, configuration, oraspect. While certain aspects of conventional technology have beendiscussed to facilitate disclosure of some embodiments of the presentinvention, the Applicants in no way disclaim these technical aspects,and it is contemplated that the claimed invention may encompass one ormore of the conventional technical aspects discussed herein. Thus, thefollowing claims are hereby incorporated into this Detailed Description,with each claim standing on its own as a separate aspect, embodiment, orconfiguration.

What is claimed is:
 1. A photovoltaic (PV) device for athermophotovoltaic system, the PV device comprising: a light absorbingstack configured to absorb radiant heat emitted from an emitteroperating at a temperature between about 1500° C. and 3000° C.; wherein:the light absorbing stack consists of: a first junction constructed witha first alloy having a bandgap between about 1.1 eV and about 1.5 eV; asecond junction constructed with a second alloy having a bandgap betweenabout 0.9 eV and about 1.2 eV; a tunnel junction positioned between thefirst junction and the second junction; and a buffer layer positionedbetween the first junction and the second junction and comprisingbetween two and 15 individual layers and having a final layer that islattice-matched to the first junction, wherein: the first alloycomprises Al_(x)Ga_(y)In_(1-x-y)As, where 0.05≤x≤0.25 and 0.45<y≤0.65,the second alloy comprises Ga_(w)In_(1-w)As, where 0.60<w≤0.80, and eachindividual layer in the buffer layer comprises Ga_(a)In_(1-a)P, where0.20≤a≤0.51.
 2. The PV device of claim 1, wherein the second alloycomprises about Ga_(0.7)In_(0.3)As.
 3. The PV device of claim 1, whereinthe tunnel junction comprises at least two of gallium, indium, arsenide,or antimony.
 4. The PV device of claim 1, further comprising asubstrate, wherein at least one of the substrate or the buffer layer isconfigured to be removed from the PV device.
 5. A thermophotovoltaicsystem comprising: an emitter operating at a temperature between about1900° C. and about 2400° C.; and a photovoltaic (PV) device, wherein:the PV device is configured to receive radiant heat from the emitter,and the PV device comprises: a light absorbing stack configured toabsorb radiant heat emitted from an emitter operating at a temperaturebetween about 1500° C. and 3000° C.; wherein: the light absorbing stackconsists of: a first junction comprising a first alloy having a bandgapbetween about 1.1 eV and about 1.5 eV; a second junction comprising asecond alloy having a bandgap between about 0.9 eV and about 1.2 eV, atunnel junction, and a buffer layer comprising between two and 15individual layers and having a final layer that is lattice matched tothe first junction; wherein: the first alloy comprisesAl_(x)Ga_(y)In_(1-x-y)As_(z), where 0.05≤x≤0.25 and 0.45<y≤0.65, thesecond alloy comprises Ga_(w)In_(1-w)As, where 0.60<w≤0.80, and theindividual layers in the buffer layer comprise Ga_(x)In_(1-x)P where0.20≤x≤0.51.
 6. A method comprising: operating an emitter at atemperature between about 1900° C. and about 2400° C. to create radiantenergy; transferring at least a portion of the radiant energy to aphotovoltaic (PV) device; and converting at least a portion of thetransferred radiant energy to electricity, wherein: the PV devicecomprises: a light absorbing stack configured to absorb radiant heatemitted from an emitter operating at a temperature between about 1500°C. and 3000° C.; wherein: the light absorbing stack consists of: a firstjunction comprising a first alloy having a bandgap between about 1.1 eVand about 1.5 eV; a second junction comprising a second alloy having abandgap between about 0.9 eV and about 1.2 eV, a tunnel junction, abuffer layer comprising between two and 15 individual layers and havinga final layer that is lattice matched to the first junction; wherein:the first alloy comprises Al_(x)Ga_(y)In_(1-x-y)As_(z)P_(1-z), where0.05≤x≤0.25, 0.45<y≤0.65, the second alloy comprises Ga_(w)In_(1-w)As,where 0.60<w≤0.80, and the individual layers in the buffer layercomprise Ga_(x)In_(1-x)P where 0.20≤x≤0.51.